USB: Add more empty functions in otg.h

Add empty functions for get/put transceiver functions too, so that
drivers that optionally use them can call them without worrying that
they might not exist, eliminating ifdefs.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>